FIELD OF THE INVENTION
The invention relates to a pulsed power supply of switched-mode power supplies, in particular of switched-mode power supplies with stand-by operation. The power supply has the following features:
a) a main converter for electrically isolating the primary and secondary circuits, as well as for transforming the voltages of the primary and secondary circuits
b) a pulsed second converter,
c) a first closed-loop control circuit for an intermediate circuit voltage, the first closed-loop control circuit being assigned to the second converter, and
d) a first output circuit connected downstream of the second converter.
Modern electronic equipment such as, for example, PC units, fax units or TV sets usually currently have a stand-by function. In the case of a TV set in the stand-by state, the set is "switched off" and can be switched again to rated duty, for example via a remote control. In the case of a fax unit, the unit is ready to receive and is switched on, for example, by an incoming fax signal. The different equipment states of stand-by operation or equipment operation therefore frequently have separate requirements for the power supply of the switched-mode power supply. The power consumption is intended to be as low as possible in the stand-by operation. In particular, in this case the aim is to supply only such equipment functions as are necessary for a quick readiness for use and which are required for detecting a start signal such as, for example, the infrared signal of a remote control or the receiving signal of a fax modem.
Having regard to as high an efficiency as possible for the operating states of the device, it is customary at present to implement the power supply by a main power supply for the nominal/maximum load and an additional auxiliary power supply for the minimum load, such as is necessary, for example, in the standby operation of a PC unit.
In addition to the known PWM converter (Pulse Width Modulation; DC/DC converter) for the main power supply of the device, the main power supplies will frequently contain a further, so-called PFC converter because of technical/economic considerations and in order to meet existing national and international standards and regulations. The PFC converter (Power Factor Correction; AC/DC converter) serves the prescribed improvement of the power factor, while the classical PWM converter undertakes the transformation and supply isolation. The power factor is defined as the ratio between absorbed real power and absorbed apparent power. The power factor is ideally equal to one. There is no reactive power in this case. For reasons of cost and because of their high efficiency, PFC converters are typically configured as step-up converters. Alternatively, the PFC converter can also, for example, be configured as a flyback converter.
It would be advantageous, with the switched off device or PWM converter section, to use the PFC converter as the power supply for the standby operation, and thus to economize on a separate auxiliary power supply. In practice, the use of the PFC converter, which appears trivial at first glance, has failed owing to the fact that because of its principle, a closed-loop control circuit must be configured to be dynamically slow for the output voltage of the PFC converter, in order to keep the modulations of the absorbed line current slight by the control. In numerous operational states, such as, for example, when running up the power supply or in the case of load steps, this leads to lengthy pauses in the pulsed operation, the result being that the flow of energy from the auxiliary power supply is interrupted. Bridging of the clock-pulse spaces, for example by larger storage capacitors, fails because the operating time is insufficient for recharging in specific operating states.
A power supply, of the generic type, of the switched-mode power supply is specified, for example, in "Power-Factor-Controller verbessert Leistungsfaktor von Schaltnetzteilen" ["Power Factor of Switched-Mode Power Supplies is Improved by the Power Factor Controller TDA 4815/19"] by Werner Schott, in the brochure entitled Siemens Components 31 (1993), Issue 2, pages 46 to 50. Specified here, in particular in FIG. 2, is a switched-mode power supply having an active harmonic filter, configured as a step-up converter (PFC converter) and a forward converter (PWM converter). Further integrated circuits of the generic type are described, for example, in the data sheets for the module TK 84 819 from the Toko company, for the module LT 1509 from the Linear Technology company and for the module ML 4824 from the Micro Linear company.